Machines are available for breaking up and excavating hard surfaces such as concrete, stone and asphalt. These machines have a rotating member, such as a wheel or a drum, with a plurality of tools located on the outer surface of the rotating member. When the rotating member is forced against the surface to be excavated, the cutting ends of the tools successively impact against the surface to be broken up, resulting in small amounts of material being removed by the impact of each tool.
The tools mounted on the rotating member have a generally concave seat at the forward end in which a tungsten carbide insert is retained. A forward cutting tip of the insert cuts into the surface to be excavated, and the useful life of the tool is determined by a number of factors. Ideally, the tip of the insert will wear evenly around its circumference and not crack or dislodge during use and, therefore, replacement will be needed only after the tool and the cutting tip are so worn as to be unusable. To maximize the resistance of the tool and the cutting tip to wear, it is desirable that the tungsten carbide cutting tip be made as hard as possible and yet not be so brittle as to break. It is also desirable that the braze which retains the tip in the seat be sufficiently strong so that the insert is not dislodged during use. The tungsten carbide insert is the most expensive portion of the manufacturing cost of such tools, and a large portion of the cost of the insert is in the raw material of which the insert is made. To be a competitive manufacturer of such tools, a manufacturing company must provide a tool having inserts that are not subject to being dislodged or cracked, and yet be competitively priced.
Currently, inserts of this type are manufactured from raw tungsten carbide powder having average particle sizes in the range of 8 to 18 microns with an average particle size midway between the extremes such that the particle distribution is in the shape of a bell curve. The raw material further includes from 6 to 11 percent by weight powdered cobalt, and after sintering, such inserts have a mean hardness which does not exceed 89.0 on the R.sub.a scale, and the hardnesses of the inserts have tolerances which are no more than.+-.0.5 R.sub.a.
When a small percentage of cobalt, such as about 6 percent, is used with smaller particles of tungsten carbide, such as less than 8 microns, the resulting product may be harder, but more brittle than presently available inserts, and would be subject to fracturing. As a result, commercially available inserts are not made from particles of raw material having average particle sizes of tungsten carbide of less than 8 microns, and present day inserts have hardnesses which do not exceed 89.0.+-.0.5 R.sub.a.
It is well known that an insert having a greater hardness would have a significantly increased resistance to wear. Even a relatively modest increase in hardness, from 89.0 R.sub.a to 89.5 R.sub.a, for example, would result in a lengthening of the life of the tool by twenty or thirty percent. Therefore, it would be desirable to provide a tool having a cemented tungsten carbide insert which has a longer usable life, without being subject to breakage. It would also be desirable to have an insert for which the cost of manufacture is reduced below existing costs.